The use of linear accelerators for the generation of a beam of either electrons or X-rays onto a target area or volume is well known. An electron gun can provide the source of electrons and after generating a stream of electrons, components in the radiotherapy machine can convert the electrons to X-rays. A flattening filter can flatten the X-ray beam, which can be further shaped to match target volume geometry with a multileaf collimator. A digital detector can be mounted and mechanically or electronically scanned synchronously with the mechanically or electronically scanned paraxial X-ray beam, providing continuous monitoring of alignment of the patient's anatomy. These systems typically provide either static fixed field radiation therapy or fully dynamic intensity modulated radiation therapy (IMRT) used by the medical community in the treatment of cancer. Advances in radiation delivery maintain the premise to maximize dose to the tumor while minimizing dose to the surrounding normal tissue. With emerging techniques to improve conformal radiotherapy, there is new emphasis on increased accuracy and reproducibility of target positioning.
Target positioning, i.e. the accurate positioning of the tumor in a radiation field, is one of the challenges inherent in radiotherapy treatment since the incorrect placement of the tumor in the radiation field is one of the most significant factors leading to the failure of local tumor-control radiation therapy. The main sources of the problem result from the fact that there is a natural motion of organs inside the body, which can range, for example, from approximately a millimeter in the case of the brain inside the skull, to several centimeters for the organs in the trunk above the diaphragm. Another factor relates to changes within an organ that can change its shape such as accepting, losing, or transferring fluids. In addition, changes to the organ can occur over the course of successful treatment, and as the tumor shrinks in volume, normal tissue, which had been displaced, returns to its original position within the volume under radiation treatment, i.e. the treatment volume.
An integrated approach is used to position the treatment volume, which consists of a gross positioning step, a coarse positioning step, and a fine positioning step. The gross positioning step can involve posture correction of the patient, while coarse positioning can locate the treatment volume relative to external body landmarks. The fine positioning step can locate the treatment volume with respect to internal landmarks, motion compensation, and gating of the treatment beam.
For example, U.S. Pat. No. 6,144,875, discusses a method of using both the coarse positioning and fine position treatment approaches to determine the position of an internal moving target region such as an internal organ, where external and internal markers (landmarks) may be used, and a model of their relative motions based on a series of images is determined prior to treatment. During treatment, little information is available on the placement of the internal landmarks except when the internal markers are periodically imaged using invasive devices, such as x-rays. Therefore, the position of external landmarks are used in real time during treatment by inferring the placement of the internal markers by referencing the pre-operative model of the relative motion of the internal and external markers. However, a problem occurs during the actual operation, namely, that it is difficult to obtain x-ray images more than once every predetermined number of seconds due to concerns about exposing the patient to too much radiation and due to the fact that the treatment beam cannot operate when x-ray imaging is being done. Here, the x-ray imaging alone would therefore be too slow to follow breathing motion with high precision without the use of external landmarks.
Traditionally, to accurately verify tumor location using the fine positioning approach, detectors such as X-ray film or electronic X-ray imaging systems are commonly used in the radiation treatment diagnostic process. In the case of electronic imaging, the megavolt therapeutic X-rays emerging from the patient can be used to generate images. However, these methods at target location deliver images of low contrast and insufficient quality. As a result, imaging with megavoltage radiation is used primarily for verification, that is, to confirm that the treatment volume is being radiated. These problems associated with utilizing high energy X-rays produced by a megavolt electron beam are the result of interacting with matter (for example, due to Compton scattering, in which the probability of interactions is proportional to the electron density).
Low energy X-rays typically have energies of about 125 peak kilovolts (kVp) or below, where a significant portion of the interactions with matter is photoelectric and the interactions are proportional to the cube of electron density. Low energy X-rays are more useful to provide accurate targeting or diagnostic information because tissue in the human body is typically of low density and as a result, the contrast achieved in low energy X-rays is far superior to that obtained with megavoltage X-rays. Therefore, distinctions of internal landmark features and the imaging of other features not perceptible with high energy X-rays are possible using kV energy. As a result, two separate imagers, each sensitive to an energy range, i.e. either the megavolt source or the kV source are used in treatment.
FIG. 1 is an illustration of a radiotherapy clinical treatment machine to provide therapeutic and diagnostic radiation, each directed to a different imager. One method taught is to have a radiotherapy machine with a therapeutic radiation source directed to a therapeutic imager along a first axis and a diagnostic X-ray source directed to a diagnostic imager along a second axis that is 90° from the first axis. This apparatus provides for the application of therapeutic radiation source capable of propagating radiation in the megavoltage (MV) energy range and for the use of kilovoltage (kV) diagnostic radiation to a separate imager. After generation of a diagnostic image by the diagnostic radiation source, the therapeutic X-ray source will rotate to the position of the diagnostic image and use the diagnostic image data for treatment of the treatment volume.
Another method taught is to incorporate a low energy X-ray source inside the treatment head of the accelerator capable of positioning itself to be as coincident with the high energy X-ray source as possible. With this approach, a high energy X-ray target is modified to include a compact 125 kV electron gun to be mounted to a moveable flange at the base of the high energy source with the cathode of the gun operably coupled to the upstream end of a drift tube. By engaging an actuator, the kV electron gun can provide radiation to a second imager that is sensitive to kV energy for providing target information. The diagnostic imager can be positioned opposite the kV electron gun with the treatment volume in between.
Therapeutic treatment can then be moved to the position used by the diagnostic imager. The therapeutic treatment beam as applied to the treatment volume can be shaped based on the data from the diagnostic imager.